Nice analysis. I tend to think the evolution of expectation of privacy from regular mail to email is similar to the evolution of expectation of privacy from land-line phones to cell phones.

That is under the 4th Amendment land-line phone users have a reasonable expectation of privacy; therefore, Gov. must obtain a search warrant to use evidence gathered from such sources against the criminal defendant. Yet, Courts define cell phones as little more than radios, and one does not have a reasonable expectation of privacy of radio transmissions, so evidence gathered can be used against a criminal defendant without being obtained by warrant.

So, right or wrong, I think the whole frame is similar to mail and email, of course emails are not defined as radio transmissions. Now all that said, between the Bush and Obama administrations reasonable expectation of privacy has been eroded (and as a result the 4th Amendment), and de facto there is no expectation of privacy over anything except maybe what is in your head.